Built new PC, plugged in USB drive and PC shut down

I built a new pc and about ten hours after running fine i plugged a flask drive into the usb slot and the computer just turned off, everything is running correctly from what i can see inside, fans, hard drive, graphics card and power supply was tested to see if it was enough power, through all the tests I have only come to the conclusion that the Motherboard might be a dud because it was the only piece i was not able to test in my friends computer swapping parts though the lights are still turning on in the motherboard if that matters or not, not sure what to do as far as fixing the problem or why the computer would just suddenly turn off right when i put the flash drive in the usb any ideas what to do?

When you power on, what does happen now?
* Any image on screen at all? Black screen with white cursor top left?
* Does the monitor light come on, then message 'no input' ?
* Can you hear the hard drive working, and LEDS flash on DVD drives?

all the lights come on and the hard drive is working along with the fans but I being new to building a computer mixed the usb hub and the fire wire leading to damage to my motherboard, do you think if i replace the damaged motherboard ill be up and running because as far as I know the CPU is fine from what i've been hearing from people with the same problem

As you know it's not easy to tell what's working and what isn't.
It's better to assume everything is working until you prove otherwise, especially if you are using new parts.
If you are using firewire and usb cards plugged into pci slots on the motherboard, power off and take them out.
If you are not sure what is working, build the board up with the minimum number of add-ons to get it running with an image on-screen.
If you can prove the motherboard is good with a minimum setup of;
C: drive , on-board graphics (if you have it) monitor, keyboard+mouse, ram plus possibly one DVD drive. If you get no image on screen, then it's could well be the motherboard is faulty.

i tried disconnecting everything from the motherboard and reconecting what you have listed in attempt to get a visual on the screen and i still got nothing for a picture is there a way to tell for sure if the motherboard is fried? at this point i dont think i have another option other then to buy another unless its a software issue

I wouldn't be surprised if the motherboard was damaged. Motherboard manuals warn that if you plug USB connectors into the firewire headers or vice versa, you will damage the motherboard, at least some that I've read. Actually, as you found out the damage doesn't occur until you plug in a device. I don't see how this could be a software issue.

yeah i did read in the motherboard manual that if you mix the firewire and usb it will result in damage to the motherboard, being that i know now that i have damaged my motherboard do you think that any other pieces in the computer could have been damaged by making this error? also do you think the motherboard is beyond repair and i should buy a new one?

It seems unlikely that other components would be damaged by your mistake but I suppose anything is possible. I doubt the motherboard is worth trying to repair even if you could find someone to do it. If I was in this unfortunate situation, I would just get another motherboard. It would save time, effort and angst to be done with it.